Justice Samuel Alito could be seen mouthing "not true" at Obama's assertion

Sen. Patrick Leahy on Thursday assailed the ruling on the Senate floor

Washington (CNN) -- The political furor escalated over President Obama's high-profile rebuke of a recent Supreme Court ruling on campaign advertising Thursday, as Democrats pounded the high court decision.

Democrats rallied around Obama the day after the president committed a rare breach of political etiquette, criticizing the controversial ruling in his State of the Union address as members of the high court sat only a few feet away.

That triggered something equally unusual. Justice Samuel Alito, a conservative backer of the ruling, frowned and appeared to mouth the words "not true." Alito's apparent reply was a rare flash of emotion among Supreme Court justices who typically sit stony faced and refrain from even clapping during State of the Union speeches.

The actions of both men may be unprecedented, according to one longtime court observer.

The court's 5-4 decision in Citizens United v. Federal Election Commission, issued last week, removed long-established legal barriers preventing corporations and unions from spending unlimited sums of money to influence voters in political campaigns. Democrats fear the decision has given the traditionally pro-business GOP a powerful new advantage.

"With all due deference to separation of powers, last week the Supreme Court reversed a century of law that I believe will open the floodgates for special interests -- including foreign corporations -- to spend without limit in our elections," Obama told a packed House of Representatives chamber Wednesday night.

Video: Leahy rips Supreme Court

"I don't think American elections should be bankrolled by America's most powerful interests, or worse, by foreign entities. They should be decided by the American people. And I'd urge Democrats and Republicans to pass a bill that helps to correct some of these problems."

Alito, part of the court's conservative majority, could be seen apparently frowning and quietly mouthing the words "not true."

Supreme Court justices rarely express any hint of emotion or opinion during the president's State of the Union speech.

On the Senate floor Thursday morning, Sen. Patrick Leahy, D-Vermont, said the ruling "goes to the very core of our democracy and it will allow major corporations -- who should have law written to control their effect on America -- instead control America. That is not the America I grew up in."

Leahy, the chairman of the Senate Judiciary Committee, ripped Alito for what he claimed was hypocrisy in preaching the virtues of judicial restraint while backing a decision overturning decades of legal precedent.

"In his confirmation hearing, Justice Alito ... testified that the role of the Supreme Court is a limited role," Leahy said. "That was then when he was seeking confirmation. This is now."

Biden called the decision "outrageous" and said "we have to correct it."

Most Republicans have defended the ruling, calling it a long overdue recognition of First Amendment rights.

Former Federal Election Commission Chairman Bradley Smith, who supports the ruling, called Obama's remarks the act of "a fairly desperate president who's trying to remake himself." In the process, Smith claimed, Obama is "willing to essentially say things that are not true."

Sen. John Cornyn, R-Texas, told reporters he thought "calling out the Supreme Court when they're sitting right there trying to impassively listen ... was a little over the top."

Lyle Denniston, a writer for the Web site Scotusblog.com who has covered the Supreme Court for five decades, told CNN he could not recall ever seeing a president rebuke the high court in such a high-profile forum. But Alito's apparent reaction, he argued, was "quite inappropriate."

Obama "was talking about the consequences of the opinion," Denniston said. Once the justices issue a decision, "they really need to let the political branches or the people deal with it as they will." Jeffrey Toobin: Alito's reaction was fair

Denniston noted that Justice John Paul Stevens, the longest-serving current member of the high court, never attends State of the Union addresses. Attending such a speech, Denniston said, involves the justices in a "political circus" that can damage a justice's image of impartiality.